If we can't figure out a good image to hint as a thumbnail for a page via
`og:image`, we set it to the reddit snoo icon. However, we have been making
this a protocol-relative url. This doesn't appear to be against [the spec][0],
but it does create problems for some scrapers.
Now we force it to be an https url, which should resolve some of those issues.
[0]: http://opengraphprotocol.org/#url